Modern wireless communications systems place great demands on the 10 antennas used to transmit and receive signals, especially at cellular wireless base stations. Antennas are required to produce a carefully tailored radiation pattern with a defined beamwidth in azimuth, so that, for example, the wireless cellular coverage area has a controlled overlap with the coverage area of other antennas. The antennas may be deployed, for example, in a tri-cellular arrangement or, 15 with a narrower beamwidth, as a six-sectored arrangement.
An antenna may comprise a single radiating structure in the form of an antenna element, or may comprise an array of antenna elements. Antenna elements may be used for reception or transmission of signals, or for both reception and transmission; an antenna element is typically reciprocal in 20 operation, that is to say it may receive or transmit with the same characteristics. An antenna element will typically be connected to a feed network having a specified terminating impedance, typically 50 Ohm, which may simply be a coaxial cable or printed track, for connecting the antenna element to other components in a radio system, such as a transmitter or receiver.
Typically cellular wireless systems employ polarisation diversity, so that it is generally required that each antenna element is capable of transmitting and receiving components of signals having orthogonal polarisations. Typically an antenna element will be arranged to receive linearly polarised components at nominally +45 degrees and −45 degrees to vertical, and each antenna elements 30 will typically have a separate feed network for signals of each polarisation.
A well known type of antenna element is the probe-fed patch antenna. Such antenna elements typically employ a radiating patch in the form of a circular or rectangular metallic conductor, which is connected to the feed network by a probe in the form of a metallic conductor. The probe is connected 5 to the patch at a feed point chosen to optimise the radiation properties for a given application. For a dual polar patch antenna element, two probes are used, each connected to the respective feed network for the polarisation, and connected to the patch at a respective feed point that will excite the desired polarisation. Typically, a probe-fed patch antenna element comprises a resonant 10 cavity formed between the patch and a ground plane. The probe may conventionally pass through the cavity for connection to a feed network on the opposite side of the ground plane from the patch.
Typically, a probe-fed patch antenna has an impedance comprising an inductive reactance, when measured at the probe. In order to connect the patch 15 to the feed network, some form of impedance matching network is typically required. This may take the form of a capacitive coupling between the probe and the patch to compensate for the reactive element of the impedance, but, depending on various factors including the size of the cavity, a transformation of real, that is to say resistive, impedance may also be required.
In some applications, such as for small base stations intended to in-fill gaps in coverage of macro-cellular base stations, it is important to constrain the size of the antenna element, in particular in terms of thickness measured perpendicular to the patch. In such an application, it may be required to use a shallow cavity, but this may require impedance matching of both complex, that 25 is to say reactive, and real, that is to say resistive, parts of the impedance of the patch to the impedance of the feed network. Resistive matching may be accomplished by an impedance matching network incorporating a transmission line of an appropriate length, but the accommodation of such a network between the probe and the connection point to the feed network will typically add to the 30 size of the antenna element, counteracting the benefit of the shallowness of the cavity.